The present application is related to electronic ballasts, and more particular to current-fed electronic ballasts designed to eliminate or minimize the striation phenomenon which can occur in gas discharge type lamps.
A gas discharge lamp converts electrical energy into visible energy by utilizing an electronic ballast to provide an alternating current flow through a gas discharge lamp. During operation of a gas discharge lamp, a phenomenon known as striations can occur. Striations can be seen in all types of gas discharge lamps, as zones of differing light intensity, causing the appearance of dark bands. This phenomenon results in an undesirable strobing effect in the lamp. In general, the lower the environment temperature, the more pronounced the striation effect. However, certain lamps will show striations at higher temperatures, including that of room temperature. This situation is particularly an issue with a newer type of energy saving lamps, which employ certain classes of gasses such as krypton.
It is well known that providing an asymmetrical current waveform through the gas discharge lamp can effectively eliminate or minimize visible striations. Based on this understanding, the lighting industry has implemented a variety of anti-striation ballast circuit configurations.
Examples of various proposed solutions include:
US2006/0103328 A1, published May 18, 2006, by General Electric, which teaches the addition of an auxiliary winding on a DC choke connected in series with the common end of the lamps to generate even harmonic current component into lamp current, to reduce or eliminate striation;
WO2006/051495A1, U.S. Pat. No. 6,756,747B2, U.S. Pat. No. 6,836,077B2, U.S. Pat. No. 4,682,082, EP852453A1, EP765107A1, teach generating an asymmetrical driver to control the two switches of the circuit, to control a flow of an asymmetrical current waveform through the lamps;
US2005/0168171A1, published Aug. 4, 2005, by an individual applicant, uses an unbalanced circuit component (an unbalanced output transformer or an unbalance DC choke) to produce asymmetric lamp current, to control striation;
US2006/0097666A1, EP547674A1, WO01/76325A1, EP1269801B1, EP1265461, teaches the addition of a striation correction circuit to inject a DC component directly into the lamp current; and
WO98/09484, published Mar. 5, 1998, by Philips Electronics, is directed to producing an asymmetric filament voltage between its opposite polarities to reduce striation, where the anti-striation circuit can be realized with low voltage components.
The above do provide various attempts to address the striation problem. However, these proposals present various disadvantages, such as but not limited to, the introduction of DC bias which leads to a shorter lamp life, as well as complicated and/or expensive circuitry. Therefore, it has been considered desirable to find an effective solution to the striation problem, without degrading the performance of the gas discharge lamp system, which also does not substantially increase the cost, particularly when used in association with energy saving high efficiency lamps.